Enable new permission dialog

How to enable the new oauth permission/install dialog? Any actual url will be helpful.

Apps Dashboard > Edit your App > Settings > Advanced > Migrations section > Enable "Enhanced Auth Dialog"

It may be because you're testing with a Facebook account that isn't logged as a developer or test user of the app. If you're using a Facebook user not associated with your app the old dialogs will still display to you.

Go to the roles section of your app, create a test user, switch to that user and start testing again.